Conditionally add json array using jq and bash.md

Query: Conditionally add json array using jq and bash

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Am trying to add a json array with just one array element to a json. And it has to be added only if it does not already exist. Example json is below.

{
    "lorem": "2.0",
    "ipsum": {
        "key1": "value1",
        "key2": "value2"
    },
    "schemes": ["https"],
    "dorum" : "value3" 
}

Above is the json, where "schemes": ["https"], exists. Am trying to add schemes only if it does not exist using the below code.

scheme=$( cat rendertest.json | jq -r '. "schemes" ')
echo $scheme
schem='["https"]'
echo "Scheme is"
echo $schem
if [ -z $scheme ]
then
echo "all good"
else 
jq --argjson argval  "$schem"  '. += { "schemes" : $schem }' rendertest.json > test.json
fi

I get the below error in a file when the json array element 'schemes' does not exist. It returns a null and errors out. Any idea where am going wrong?

null
Scheme is
["https"]
jq: error: schem/0 is not defined at <top-level>, line 1:
. += { "schemes" : $schem }                   
jq: 1 compile error

Edit: the question is not about how to pass on bash variables to jq.

A:

using jtc, it's quite simple:

bash $ schem='["https"]'
bash $ 
bash $ <example.json jtc -w'<>f[schemes][-1]' -i'{"schemes":'$schem'}"'
{
   "dorum": "value3",
   "ipsum": {
      "key1": "value1",
      "key2": "value2"
   },
   "lorem": "2.0",
   "schemes": [
      "https"
   ]
}
bash $ 

- insert would fail if "schemes" exists because inserting a clashing label with -i always fails - it always preserves the existing one
- insert would work if "schemes" is missed because upon trying to walk [schemes] walking would fail and the walk-path would get reinstated at root (thanks to <>f lexeme) then the whole object under insert option {\"schemes\":$schem} would get inserted

Naturally, if in-place modification of the source file is required - use -f option:

bash $ jtc -w'<>f[schemes][-1]' -i'{"schemes":'$schem'}"' -f example.json